Exercise 1
----------
Too many parens in set-variable-value!'s let:

 (define (set-variable-value! var val env)
  (let ((binding ((find-in-environment var env))))
;                 ^                           ^


Exercise 2
----------
'or' doesn't evaluate its arguments:

(or (= 1 2))
;; => (= 1 2)

It also defaults to true, instead of false:

(or)
;; => #t

Change 'eval-or' to actually run the first clause through 'm-eval'

(Students may note and fix exercise 4 while they're here, 'or' evaluating the
argument only once)


Exercise 3
----------
The not primitive is defined incorrectly:

        (list '= =)
        (list 'display display)
        (list 'not 'not)
;                  ^
;                No quote wanted here

Exercise 4
----------
'or' should only evaluate its argument once.

'eval-or' should use a 'let' to temporarily store the result of
m-eval'ing the first clause.


Exercise 5
----------
Extend-environment adds to the wrong side of the environment stack:

(define (extend-environment vars vals base-env)
  (if (= (length vars) (length vals))
      (append base-env (list (make-frame vars vals)))
;             ^--(1)-^ ^-----------(2)-------------^

(1) and (2) should be swapped.